Flathead frustration - "pinging"

I'm guessing Jonnyzepp has fixed his problem by now, but I'll add what happened to me. I had a horrible pinging problem on a built 59A with a '42 distributor. Worried about compression, timing, gas, and everything else I could think of. Turned out that the leather on the end of the advance brake was worn off. Don't discount the importance of that little plunger.

 

The interference method I have posted on here several times allows a very close determination of TDC with heads on and just a few minute's work...just do it and quit fumbling in the outer darkness!
Meanwhile, cruise pinging may well relate to the brake, as Alchemy noted, full throttle is entirely under control of the centrifugal governor and should not be excessive. Full advance on slot may well have moved you away from proper curve, most like close to the stock static timing which is likely to be real close to center of slot range...